2008AJ....136..566G

I present details of the variations of several hundred red giant stars on timescales of a few hours to a few days from Hubble Space Telescope (HST) observations of a low-extinction galactic bulge sample from an intensive seven-day campaign. Variations in the red giants are shown to be a strong function of position within the color-magnitude diagram (CMD) in accord with general expectations from theory. Amplitudes are greater for stars with larger radii, whether this results from higher luminosity at the same effective temperature or lower temperature at a fixed apparent magnitude. Likewise, characteristic timescales for the variations increase to the upper right in a CMD as does the ratio of amplitudes measured at 606 nm compared to 814 nm. Characteristic variation timescales are well matched by low-order radial pulsation modes. The effective sample discussed here extends from about two magnitudes above the bulge turnoff at which red giant radii are ∼7 R/R_☉ at 5000 K with typical amplitudes of ∼0.5 mmag to ∼40 R/R_☉ at 4000 K with amplitudes of ∼3.5 mmag. Variability characteristics are quite similar at any given position in the CMD, and at levels in the CMD where oscillations are easily detected nearly all red giants show such. If these variations represent oscillations with sufficient lifetimes to derive accurate mode frequencies, more extensive observations, e.g., as should soon be provided by the Kepler Mission, would provide a rich asteroseismic return.